1. Field
The invention is in the field of measuring relative motion between two points, such as between two separate items or between two parts of the same item, and particularly, accurately measuring small amounts of such relative motion.
2. State of the Art
Various devices and methods have been developed for measuring relative motion between two items or between two parts of the same item. Most of these are limited to measuring relative motion in a limited number of directions or planes. For example, relative motion may be measured in a single direction for items which move toward or away from one another by measuring the distance between the points at any particular time by using measuring equipment such as rulers or calipers. Such measuring equipment does not work where the relative movement to be measured occurs or may occur in several dimensions, or where measurements are to be taken during movement. In measuring relative movement of various body parts or in measuring relative movement of two parts of a item when the item is subjected to strain or stress, specialized measuring equipment is required.
Several devices have been developed to measure movement of the jaw and mandibular joint which use light sources and light detectors to sense such movement. U.S. Pat. No. 4,447,207 shows three light sources with three associated detectors. The three light sources are spaced in fixed relationship and attached to move with the jaw of the user as the jaw is moved in relation to the user's head. The detectors are held fixed with a user's head and measure the linear movement of the light sources in two dimensional planes as the jaw is moved relative to the head. The measurements are not used to actually measure relative movement between the jaw and head, but are used to reproduce in a model joint the same movement as detected in the movement of an actual jaw.
U.S. Pat. No. 4,496,952 shows a device for measuring jaw movement wherein movement of the jaw is transmitted to a measuring device including a plurality of light sources and an equal plurality of light position sensors. In several embodiments, three light sources generate three mutually orthogonal, but offset beams of light. These beams are directed to three mutually orthogonal reflecting grids. Light sensors are located adjacent the light sources and receive light reflected from the light reflecting grids. The sensors generate pulses, the number of pulses generated being proportional to the movement of a light beam in one dimension along the grid. In another embodiment, two mutually orthogonal, but offset, light beams are generated and directed to two orthogonal sensing devices which sense the position and movement of each beam in two dimensions. The patent teaches that three dimensional movements can be measured using three one dimensional grids, or using two two dimensional grids. However, the device of U.S. Pat. No. 4,495,952 will measure only translational movement in three dimensions, it will not account for rotational movement of the light sources and, if rotational movement occurs, it will provide incorrect measurements of movement and position of the light sources. Thus, the device of U.S. Pat. No. 4,495,952 can be used only where steps are taken to insure that no rotational movement is present.
In many areas of mechanical testing it is desired to measure relative movement between two items or to measure the relative movement between two parts of the same item. The relative movement usually occurs in three dimensions and includes rotational movement within those three dimensions. There is currently no apparatus or method known to the inventor for accurately measuring the relative movement where completely unrestrained three dimensional relative movement between two points is allowed. Such measurement would be desirable in many testing situations. For example, arthrodesis, a procedure for treating spinal back problems by causing the vertebra to grow together or fuse to form an immobile joint or unit, is a common orthopedic procedure. It is generally thought to be important to immobilize the vertebra involved to allow the vertebra to fuse. If excessive relative motion between the two vertebra occurs, the vertebra may not successfully fuse. In order to maintain an immobilized or motionless state during the healing or fusing process, it is usual practice to implant an immobilization device and secure it to appropriate vertebra. The purpose of the immobilization device is to keep the adjacent vertebra to be fused motionless until the two vertebra have fused. However, it has been difficult to test such immobilization devices to determine their real effectiveness. Strength and failure testing on the devices themselves do not indicate how well they actually perform to immobilize the attached or intermediate vertebra. Past testing of immobilization devices affixed to cadaver spinal columns have been useful in determining the effectiveness of the attachment of the device to the vertebra, but have not been effective in measuring the effectiveness in actually immobilizing specific vertebra. This is because relative motion of the vertebra has been impossible to accurately measure with existing measuring devices. Similarly, unrestrained, three-dimensional relative movement has been difficult to accurately measure in various other biomechanical, mechanical, or stress or strain analysis situations.